I know it makes me happy because it's such a phenomenon with kids in the house! Usually I open the lid to find just enough peanut butter left at the bottom to warrant letting the dog clean it out before tossing it into the recycle bin.
I know! The new jar is such a fleeting feeling of abundance. :-)
It's also like that first step you take after a new snowfall. There's such an unbroken purity, and (maybe I'm going a little too far with this but...) a sense of oncoming adventure!